The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to attenuating the effect of rotational vibration on the positioning of a read/write head relative to a disc surface.
Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position. A servo system samples the position of the read/write head relative to a particular track at a particular sampling rate and adjusts the position of the head.
In a typical servo system, the actual position of the read/write head relative to a given track is sensed and compared to the desired position of the head. A position error signal (PES) indicative of the difference between the actual and desired positions is provided to a servo controller. Based on the value of the position error signal, the servo controller provides a servo control signal to a power amplifier that amplifies the servo control signal and provides it to a voice coil motor. The voice coil motor is coupled to an actuator that moves in response to the application of the amplified control signal to the voice coil motor. An actuator arm that holds the read/write head is coupled to the actuator. In this way, the servo controller controls the positioning of the read/write head relative to a particular track on the disc surface.
As the areal density of magnetic disc drives increases, so does the need for more precise position control when track following, especially in the presence of external vibrations. In order to desensitize the disc drive to translational vibrations, a balanced mechanical actuator has been typically used. However, since the actuator must pivot freely to access the data, the effects of rotational vibration about the axis parallel to the pivot axis can be considerable. Such rotational vibrations are commonly caused by vibrational energy produced by another voice coil motor/actuator assembly in the same drive when said other actuator assembly is seeking from one track to another. Such rotational vibrations can cause non-repeatable runout (NRRO) of the position error.
A number of researchers have investigated the feasibility of using accelerometers for disturbance rejection. Hanks proposed using accelerometers mounted to the disc-drive assembly to attenuate disturbances. (U.S. Pat. No. 5,299,075). A single constant gain, which was determined off-line, was used to feed the accelerometer signal to the voice coil motor. Abramovitch used the least-mean-square (LMS) adaptation algorithm to estimate the value of the single gain. No plant estimation of the disc drive was attempted. Abramovitch, Daniel Y., xe2x80x9cRejecting Rotational Disturbances on Small Disk Drives Using Rotational Accelerometers,xe2x80x9d Control Engineering Practice, vol. 5, no. 11, November 1997, p 1517-1524. Kempf used an accelerometer on a compact disc player to control the focus length of the reading lens. The filtered-x LMS adaptation algorithm was applied to the controller parameters. Kempf, C. J., Design of Servo Systems for Disturbance Rejection and Applications to Disk File Storage Systems, Ph.D. thesis, University of California at Berkeley, 1994.
The present invention provides a solution to these and other problems and offers other advantages over the prior art.
The present invention serves to attenuate the effect of rotational vibration on the positioning of a read/write head relative to a disc surface.
One embodiment of the present invention is directed a method of attenuating the effect of rotational vibration on the positioning of the read/write head. Pursuant to the method, the rotational acceleration of the disc drive body is measured. A rotational acceleration signal indicative of the measured rotational acceleration is provided to an adaptive filter. The adaptive filter is adapted to further receive the position error signal and to produce a feedforward signal calculated to offset the disturbance to the read/write head position caused by rotational vibration. The adaptive filter further adjusts its parameters based upon the rotational acceleration signal, the position error signal and a transfer function relating an actual position signal, indicative of the actual position of the read/write head relative to the disc surface, to the feedforward signal. The feedforward signal is provided to the servo loop. The transfer function relating the actual position signal to the feedforward signal is estimated off-line and stored in memory for access by the adaptive filter during operation.
Another embodiment of the present invention is directed to a disc drive servo system that includes an actuator assembly, a servo controller, a rotational acceleration sensor, an adaptive filter and a computer storage medium. The actuator assembly positions a read/write head relative to a disc surface. The servo controller receives a position error signal indicative of a difference between an actual position of the read/write head relative to the disc surface and a desired position of the read/write head. The servo controller then provides a servo control signal to the actuator assembly based on the value of the position error signal. The rotational acceleration sensor measures the rotational acceleration of the disc drive body and produces a rotational acceleration signal indicative of the measured rotational acceleration. The adaptive filter receives the rotational acceleration signal and the position error signal and provides a feedforward signal to the actuator assembly. The feedforward signal is adapted to offset the disturbance to the read/write head position caused by rotational vibration. The adaptive filter adjusts its parameters based upon the rotational acceleration signal, the position error signal and a transfer function relating an actual position signal, indicative of the actual position of the read/write head relative to the disc surface, to the feedforward signal. The computer storage medium holds a predetermined plant estimate of the transfer function relating the actual position signal to the feedforward, the plant estimate being determined off-line.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.